It has previously been known how to cast framework with the aid of an integral form consisting of cassettes arranged next to one another, whereby the underside of the form constitutes a complete ceiling in the room under the framework. Such a cassette consists of a U-shaped, longitudinal unit of thin steel sheet comprising a rectangular bottom with an edge beam section extending along its respective two parallel longitudinal sides, substantially at a right angle to the bottom on the side which is to be turned up. The said edge beam section comprises both a web rising at a right angle from the bottom and at its upper edge a flange extending at a right angle away from the web and towards the centre on one side of the cassette and away from the centre on its other side. This imparts to the cross-section of the edge beam section a shape enabling adjacent cassettes to be hooked into one another so as to form a continuous framework. The flanges are at their free edges provided with a support edge facing downward, which ensures additional stability and prevents two adjacent units from sliding apart during the casting process.
When casting a framework with the known cassettes by way of a form, their ends are made to rest on the shell of the building, which may consist of a steel shell, temporary support beams resting on temporary piles supporting it along its length. The cassettes in the form are hooked to one another in sequence until the entire area of the framework has been covered. Once the adjacent webs have been joined for instance with the aid of rivets the concrete is applied.
The webs of the edge beam section are provided with holes and aligned with one another in two adjacent edge beam sections, as a result of which the concrete forms a connection along the length of the beams.
The cassettes are subject to tensile forces acting in their plane both during casting and after hardening and loading of the concrete. It is therefore important that the cassettes should be joined with great accuracy. This has proved to be difficult with previously known designs. Apart from a reduced capacity to absorb such tensile stresses the cassettes had a tendency to separate owing to indentations about the rivet heads but also because the fasteners were located too far from the undersides of the cassettes. Furthermore, if a fire broke out, the edge beam sections used also to be in part directly exposed to heat, which is normally not permissible, unless special measures were taken.
Nor are these designs proof against the escape of unbound concrete water while the concrete is being cast, such water being able to flow down on the undersides of the cassettes causing such disfigurations that costly measures could not be avoided.